This invention relates to a gear made of fiber reinforced resin or a resin gear, and more particularly to a gear constituted by a bushing made of metal and a gear body which includes a core made of a reinforcing fiber substrate and formed into a ring-like shape and is integrally arranged on an outside of the metal bushing.
There has been conventionally known in the art techniques of forming resin into a gear body by compression molding while using a reinforcing resin substrate made of an aramid fiber as a core. For example, techniques of preparing a prepreg by previously impregnating a sheet-like non-woven fabric made of a blend material of a para-type aramid fiber and a meta-type aramid fiber with phenolic resin, rounding the prepreg into a bar-like shape and mating or matching both ends of the thus-rounded prepreg with each other to form the prepreg into a ring-like shape, resulting in providing a reinforcing fiber substrate are disclosed in Japanese Patent Application Laid-Open Publication No. 113458/1995. Unfortunately, the reinforced fiber substrate constituted of such a non-woven fabric fails to exhibit enhanced mechanical strength. Also, the prepreg causes flowing of the resin impregnated therein, leading to deformation of the reinforcing fiber substrate, so that the reinforcing fiber substrate spreads to a portion of the metal bushing positioned in proximity to a corner thereof defined between an outer peripheral surface of the metal bushing and a side surface thereof. This causes the reinforcing fiber substrate and resin to project to the side surface of the metal bushing, although it permits an increase in bonding strength between the gear body and the metal bushing.
In order to solve the problem, techniques that a cylindrical body made of a woven or knitted fabric of an aramid fiber yarn (a blended yarn or union yarn of a meta-type aramid fiber yarn and a para/meta-type aramid fiber yarn) is used as a material for a reinforcing fiber substrate are proposed, as disclosed in Japanese Patent Application Laid-Open Publication No. 156124/1996. In the techniques proposed, the cylindrical body is bent into a ring-like shape along a central line thereof from one or both ends of the cylindrical body, so that the reinforcing fiber substrate is provided. Then, the reinforcing fiber substrate is placed in a molding die together with a metal bushing while keeping the metal bushing arranged at a central position of the reinforcing fiber substrate. Thereafter, liquid thermosetting resin such as epoxy resin, crosslinked polyamino amide or the like is poured into the molding die reduced in pressure, to thereby be impregnated in the reinforcing fiber substrate. Subsequently, the molding die is heated to a predetermined temperature, so that a resin molded member which includes the reinforcing fiber substrate having the resin impregnated therein and acting as a core is integrally formed on an outside of the metal bushing by cast molding. Then, the resin molded member is subjected to cutting, to thereby be formed on a periphery thereof with teeth, resulting in a resin gear being provided. The thus-manufactured resin gear is so constructed that the reinforcing fiber substrate is formed into a continuous ring-like shape without any joint. Such construction permits the gear to entirely exhibit uniform strength. Also, in Japanese Patent Application Laid-Open Publication No. 156124/1996 described above, techniques of subjecting the above-described blended yarn or union yarn to plain weaving, to thereby prepare a sheet-like fabric and winding the fabric into a bar-like member, of which both ends are matched with each other to provide a ring-like reinforcing fiber substrate are disclosed as a comparative example as shown in FIG. 2 thereof The reinforcing fiber substrate thus obtained is placed in a molding die, resulting in a resin molded member being formed by compression molding in a manner like the above.
Further, additional techniques are proposed as disclosed in Japanese Patent Application Laid-Open Publication No. 299865/1998, wherein such a cylindrical body as described above is made of a woven fabric and formed into a shape like a bellows. Then, the thus-formed cylindrical body is wound up from one end thereof, to thereby be formed into a ring-like shape. The woven fabric is made of a blended or union aramid yarn constituted by a meta-type aramid fiber and a para-type aramid fiber. More detailed description of the fibers is not made in the publication. In the techniques disclosed, a ring-like reinforcing fiber substrate is placed in a temporary molding die prior to arrangement thereof in a molding die, resulting in being formed into a predetermined configuration.
The above-described aramid fiber yarn obtained by blending of the meta-type aramid fiber and para-type aramid fiber and used for the cylindrical body permits the gear made of resin to be increased in strength and durability. However, an increase in proportion of the para-type aramid fiber in the aramid fiber yarn causes cutting operation for formation of teeth of the gear to be deteriorated, although it increases strength of the gear as described above. Also, it tends to cause the aramid fiber yarn to be left on a cut surface of the teeth without being cut out during the cutting operation. Further, the aramid fiber yarn thus having failed to be cut out is caused to be pulled during the cutting operation, to thereby be drawn out of the reinforcing fiber substrate, resulting in the reinforcing fiber substrate and resin being separated or peeled from each other at an interface therebetween. Such peeling causes moisture or the like to intrude into the resin gear, leading to a deterioration in durability of the resin gear.
The present invention has been made in view of the foregoing disadvantage of the prior art.
Accordingly, it is an object of the present invention to provide a gear made of fiber reinforced resin or a resin gear which is capable of exhibiting enhanced mechanical strength and service durability while optimizing a construction of a reinforcing fiber substrate formed into a ring-like shape.
It is another object of the present invention to provide a gear made of fiber reinforced resin or a resin gear which is capable of enhancing bonding strength between a metal bushing and a resin molded member.
In accordance with one aspect of the present invention, a resin gear or a gear made of fiber reinforced resin is provided. The resin gear includes a metal bushing and a gear body made of resin and arranged on an outside of the metal bushing. The gear body includes a resin molded member integrally made of a material containing thermosetting resin as a part thereof and formed on an outer periphery thereof with a plurality of teeth. The resin molded member is formed by cast molding while using at least one reinforcing fiber substrate formed into a ring-like shape as a core thereof. The reinforcing fiber substrate is constituted by a fabric selected from the group consisting of a knitted fabric of a union yarn made of a para-type aramid fiber and an organic fiber reduced in strength as compared with the para-type aramid fiber and a woven fabric thereof.
In the present invention, the reinforcing fiber substrate has a volume percentage of 30% to 65% by volume based on a volume of the resin molded member. Also, the para-type aramid fiber has a mass percentage of 30% to 60% by weight based on a mass of the union yarn.
In a preferred embodiment of the present invention, the reinforcing fiber substrate is constituted by the knitted fabric and the reinforcing fiber substrate has a volume percentage of 30% to 65% by volume based on a volume of the resin molded member.
In a preferred embodiment of the present invention, the ring-like shape of the reinforcing fiber substrate may be obtained by winding up a cylindrical body made of the knitted fabric along a central line thereof from at least one end thereof. The reinforcing fiber substrate may have a volume percentage of 45% to 55% by volume based on a volume of the resin molded member. Alternatively, the ring-like shape of the reinforcing fiber substrate may be obtained by matching or mating both ends of a bar-like member formed by winding the knitted fabric with each other, wherein the reinforcing fiber substrate has a volume percentage of 45% to 55% by volume based on a volume of the resin molded member.
In a preferred embodiment of the present invention, the ring-like shape of the reinforcing fiber substrate may be obtained by winding up a cylindrical body made of the woven fabric along a central line thereof from at least one end thereof. The reinforcing fiber substrate may have a volume percentage of 40% to 65% by volume based on a volume of the resin molded member. Alternatively, the ring-like shape of the reinforcing fiber substrate may be obtained by matching both ends of a bar-like member formed by winding the woven fabric with each other, wherein the reinforcing fiber substrate has a volume percentage of 40% to 65% by volume based on a volume of the resin molded member.
When each of the knitted fabric and woven fabric is used for the reinforcing fiber substrate, the para-type aramid fiber preferably has a mass percentage of 40 to 50% by mass based on a mass of the union yarn.
Such formation of the reinforcing fiber substrate as described above in the present invention permits the reinforcing fiber substrate to be deformed by a pressure produced due to closing of the molding die at the time of cast molding, resulting in the reinforcing fiber substrate being fitted on the outer periphery of the metal bushing. Also, the above-described construction of the reinforcing fiber substrate prevents the reinforcing fiber substrate from projecting beyond a corner between a side surface thereof and the outer periphery of the metal bushing when the liquid resin is poured into the molding die and when the resin is cured. This keeps the reinforcing fiber substrate from being partially lacking at a joint interface between the resin molded member and the metal bushing, to thereby ensure uniform joint strength therebetween.
A mass percentage of the para-type aramid fiber defined within the above-described range not only ensures the above-described advantage, but eliminates a disadvantage that the aramid fiber yarn is left on a cut surface of the teeth without being cut out during the cutting operation, to thereby be drawn out of the reinforcing fiber substrate, resulting in the reinforcing fiber substrate and resin being separated or peeled from each other at an interface therebetween. Also, it permits the para-type aramid fiber to sufficiently exhibit increased strength, to thereby ensure both satisfactory processing characteristics and increased mechanical strength of the reinforcing fiber substrate.
Thus, when a volume percentage of the reinforcing fiber substrate is below the above-described lower limit, the resin gear of the present invention fails to prevent a region occupied by only the resin from being present in the resin molded member, whereas a volume percentage of the reinforcing fiber substrate above the above-described upper limit causes the resin to be insufficiently impregnated in the reinforcing fiber substrate. When the knitted fabric is used for the reinforcing fiber substrate, the reinforcing fiber substrate is readily expanded or stretched by a pressure produced due to closing of the molding die because the knitted fabric is stretchable. This permits the reinforcing fiber substrate and resin to be distributed in the resin molded member as uniformly as possible. Thus, formation of the knitted fabric into the reinforcing fiber substrate permit a range of a volume percentage of the reinforcing fiber substrate to be enlarged as compared with that by the woven fabric.
In a preferred embodiment of the present invention, the metal bushing preferably includes a cylindrical body and a detachment-preventing section integrally provided on a central portion of an outer periphery of the cylindrical body in a manner to extend in a radial direction of the cylindrical body. The detachment-preventing section of the metal bushing includes a plurality of recesses and a plurality of projections arranged alternately in a peripheral direction of the cylindrical body. The recesses each are formed into an arcuate shape or a shape curved so as to project inwardly in the radial direction of the cylindrical body as viewed in a direction perpendicular to the radial direction and peripheral direction. Two such reinforcing fiber substrates are superposed on each other in a manner to interpose the detachment-preventing section of the metal bushing therebetween and be contacted at a portion thereof positioned outwardly in a radial direction of the detachment-preventing section with each other. The recesses and projections cooperate with each other to prevent two such reinforcing fiber substrates from moving or rotating around the metal bushing. Also, the thus-constructed detachment-preventing section prevents the reinforcing fiber substrates from detaching from the metal bushing. In particular, formation of the recesses into the above-described configuration permits a part of each of the reinforcing fiber substrates to smoothly enter the recesses along a curved surface of the recesses, to thereby ensure that the reinforcing fiber substrates may be satisfactorily filled in the recesses. The recesses each have no angular or sharp portion formed therein, to thereby prevent each of the recesses from being formed therein a resin-rich portion which is formed of only the resin without being filled with the reinforcing fiber substrate. This eliminates a deterioration in bonding strength between the metal bushing and resin molded member.
In particular, the projections each may be formed into an undercut shape which permits a width of the projection in cross-section defined in a direction perpendicular to the peripheral direction to be gradually increased outwardly in the radial direction. This further enhances both detachment-preventing and rotation-preventing functions of the detachment-preventing section. Further, in a preferred embodiment of the present invention, the projections each may be so formed that the smallest distance d between an intersection between a virtual line defined so as to extend from an edge of a top surface of the projection toward a center of the cylindrical body and the outer periphery and a line formed due to intersection between a side surface of the projection and the outer periphery is within a range of between 30% and 85% based on a thickness h of the projection in the radial direction. Such configuration exhibits various advantages. More specifically, rotation of the gear leads to generation of stress which tends to cause the projections to shear the resin molded member. Formation of each of the projections into the undercut shape permits an increase in area of the resin molded member at a joint between the metal bushing and the resin molded member, leading to an increase in resistance to stress of the resin molded member. Resin is generally increased in thermal expansion coefficient as compared with metal. Thus, an increase in temperature causes generation of stress which tends to separate the resin molded member from the metal bushing due to a difference in thermal expansion therebetween. However, formation of the projection into the undercut configuration effectively eliminates the above-described separation of the resin molded member from the metal bushing. When the smallest distance d is below 30% based on the thickness h of the projection, the above-described advantage of increasing the resistance to stress is deteriorated; whereas the distance d above 85% of the thickness h leads to a reduction in mechanical strength of a base of the projection.
In accordance with another aspect of the present invention, a method for manufacturing a gear made of fiber reinforced resin is provided. The method includes the step of arranging a metal bushing and a ring-like reinforcing fiber substrate in a molding die while positioning the reinforcing fiber substrate outside the metal bushing. The reinforcing fiber substrate is made of a fabric selected from the group consisting of a knitted fabric of a union yarn made of a para-type aramid fiber and an organic fiber reduced in strength as compared with the para-type aramid fiber and a woven fabric thereof. The method further includes the steps of pouring thermosetting resin into the die which is previously heated, followed by application of heat to the die to prepare a resin molded member integrated with the metal bushing and forming a plurality of teeth on an outer periphery of the resin molded member. The reinforcing fiber substrate has a volume percentage of 65% by volume or less based on a volume of the resin molded member and the para-type aramid fiber has a mass percentage of 30% to 60% by weight based on a mass of the union yarn. The method further includes the step of pressing the reinforcing fiber substrate into a configuration approaching to that of the resin molded member formed by the die prior to arrangement of the reinforcing fiber substrate in the die.
Receiving of the reinforcing fiber substrate in the molding die causes the reinforcing fiber substrate to be hard to move therein. Thus, the above-described arrangement of the reinforcing fiber substrate in the molding die after pressing of the reinforcing fiber substrate into a shape approaching to that of the molding die prevents the gear from having a portion occupied by only the resin. Also, it minimizes projection of the reinforcing fiber substrate to an upper surface of the metal bushing.